1. Technical Field of the Invention
The present invention relates generally to a hydraulic control valve equipped with a piezoelectric valve actuator and a fuel injector using the same.
2. Background Art
Typical common rail fuel injection systems for diesel engines of automotive vehicles have a common rail in which the high-pressure fuel supplied from a high-pressure pump is stored and inject the high-pressure fuel into each cylinder of the engine through a fuel injector. In recent years, as such fuel injectors, hydraulic control valves equipped with a quick response piezoelectric valve actuator are proposed which include a large-diameter piston moved by the expansion and contraction of the piezoelectric valve actuator, a displacement amplifying chamber filled with hydraulic fluid, and a small-diameter piston which are arranged in alignment with each other. The movement of the large-diameter piston causes the hydraulic fluid in the pressure chamber to change in pressure, which moves the small-diameter piston, thereby actuating a control valve. Specifically, the displacement amplifying chamber works to hydraulically amplify the expansion or displacement of the piezoelectric valve actuator using hydraulic leverage and transmit it to the small-diameter piston. The amplification factor is expressed by a ratio (S/s) of a pressure-energized area (S mm2) of the large-diameter piston (i.e., an area of an end of the large-diameter piston on which the hydraulic pressure acts) to a pressure-energized area (s mm2) of the small-diameter piston.
The control valve is designed to selectively close one of a low-pressure port leading to a drain passage and a high-pressure port leading to the common rail to control the pressure within a control chamber providing a back pressure to a nozzle needle of the fuel injector. Specifically, when the control valve opens the low-pressure port to establish fluid communication between the control chamber and the drain passage and closes the high-pressure port, it will cause the pressure in the control chamber to drop to lift the nozzle needle upward, thereby spraying the fuel from a spray hole. When it is required to terminate the fuel injection, the control valve opens the high-pressure port to establish fluid communication between the control chamber and the common rail while it closes the low-pressure port, it will cause the pressure in the control chamber to rise to move the nozzle needle downward, thereby closing the spray hole.
Typical piezoelectric devices used as actuators have a displacement-to-output force relation, as shown in FIG. 4(a), when an applied voltage is constant (a maximum allowable voltage). The maximum allowable voltage is a recommended maximum voltage or working voltage (e.g., 150V) which can be applied in safety to the piezoelectric device without danger of breakdown of the piezoelectric device or an electric driver therefor. If the applied voltage is constant, a mechanical distortion or displacement of the piezoelectric device is produced in inverse proportion to the output force thereof. When the displacement of the piezoelectric device is suppressed completely, the piezoelectric device produces a maximum output force. A work of the piezoelectric device is proportional to an applied electric energy and has a given correlation to the output force. The relation between a maximum work and output force of the piezoelectric device when subjected to a maximum load is shown in FIG. 4(b). In general, it is known that an actuator implemented by a piezoelectric device is preferably all designed in terms of energy efficiency so that the output force of the actuator produced when undergoing a maximum required load may be one-half of a maximum possible output force thereof (i.e., an output force produced when the deformation of the actuator is restricted to zero), thereby allowing a maximum work to be obtained under application of a constant electric energy to the actuator.
Designing a fuel injector to have the above structure using the hydraulic leverage, we have encountered the following drawback. The fuel injector experiences a maximum required load when the control valve opens the low-pressure port. The output force F of a piezoelectric actuator required to open the low-pressure port is expressed by F=SL·P·(S/s) where SL is an area (mm2) of a plane defined by an annular line that is a line of contact between the control valve and a valve seat around the low-pressure port (which will be referred to as a seat area below), P is the pressure (Kg/mm2) of fuel within the common rail, and S/s is the amplification factor. The amplification factor was so determined that the output force F might be one-half of a maximum output force, but the amount of lift of the control valve did not reach a target value expected from FIGS. 4(a) and 4(b). The same problems was posed in a case where the output force required to close the high-pressure port (usually lower than the output force required to open the low-pressure port) was determined like the above.
Analyzing the above problem, we have found that the factor that the amount of lift of the control valve did not reach the expected target value is the structure of the fuel injector itself. Specifically, when an electric energy is applied to the piezoelectric actuator, it will cause the hydraulic pressure in the displacement amplifying chamber to rise through the large-diameter piston. When the hydraulic pressure within the displacement amplifying chamber acting on the small-diameter piston exceeds the pressure of fuel exerted from the high-pressure port on the control valve, it initiates a lift of the control valve to open the low-pressure port. At this time, the electric energy applied to the piezoelectric actuator is so controlled as to bring the voltage developed across terminals of a piezoelectric element of the piezoelectric actuator (will also be referred to as a piezo voltage below) into agreement with the above maximum allowable voltage. However, electric charges proportional to the output force are produced in the piezoelectric element when the control valve is lifted up, so that the energy applied to the piezoelectric actuator is decreased by an amount of energy of the charges. Therefore, when the output force of the piezoelectric actuator is decreased by a drop in hydraulic pressure within the control chamber caused by the lift of the control valve, the charges disappear from the piezoelectric element, so that the piezo voltage drops. This voltage drop has been found to be the factor that the amount of lift of the control valve does not reach the expected target value.